Make YOUR Own Portable Bluetooth Speaker From Scratch

In this instructable i will guide you through the steps of making a portable (bluetooth) speaker from the ground up.

It might sound like a hard project for the uninitiated, so my goal here is to help anybody who wants to try this by explaining things as simple as possible, without missing any important step. Let's begin!

The most basic tools i consider necessary for making a speaker are a drill and a jigsaw, alongside with appropriate blades, drill bits (i also suggest a step drill bit) and measurement tools. Yes, you can get straight cuts with a jigsaw by using some straight wood pieces as side guides; obviously if you want to step up your equipment, a table saw and a router will be your best friends (which i still don't own though). You will also need a soldering iron and some tin to solder all the wires.

Other supplies you will need are: copper wire (red and black speaker wire is okay, black and red will help distinguishing positive and negative easily), screws (wood or metal screws depending on materials), wood glue (for wooden speakers), a glue gun and silicone (for attaching and sealing things), (optional) a heat gun (in case you want to flare and bend pvc tubes used as bass reflex ports), an on-off switch, a 2.1x5.5 (or preferred size) barrel jack (depends on the charging system), some kind of protective feet (i use rubber ones), a handle, some LEDs if you want.

You should start from determining how much money you want to invest in the project, as staying in budget requires some balance (you can't spend 90% of the budget on speakers alone, as you won't have any for the rest).

If your budget is very tight, my recommendations is to reuse what you already have: old speakers or entire speaker boxes from micro-hifi systems, salvaged batteries (more on that in the battery section), leftover wood.

You can choose basically anything as a material for a speaker box, but obviously not all materials are adequate, for various reasons.

My recommendation is to go with plywood: choose a variety that has a good compromise between stiffness and weight, because both are equally important in a portable design. You can either paint it with a primer, sand and then spray-paint to get the colour you want, or simply use wood paints to keep the wooden look.

Another widely used material is MDF, which is a good compromise all things considered. It must be treated with a sealant and not used as-is because moisture destroys it. As for the thickness, i'd say the same as above: not too thick because of weight, not too thin because the panels will vibrate. I tend to use plywood between 10 and 20mm thick.

In case you find the material you chose to vibrate a bit, you can always reinforce it by doing internal braces.

So-so materials: metal (easy to work with but vibrates if thin, for ammo-box speakers i'd recommend building an internal wooden box), particle board (wouldn't buy it on purpose, but it can be used if you already have it), plastic (kinda like metal, plus all commercial offerings are plastic, why not make something different? ;) )

I attached some pics of painted plywood (made by my woodworker cousin), treated plywood and a metal box for reference.

Build your box the way you prefer, be sure to secure everything with glue and nails or screws.

This is a trivial part: what kind of speakers should I use? What if I don't know much about speaker building in general?

Let's start with car audio speakers (coaxials, 2 way, not subwoofers). These are very appealing to anybody who doesn't want to invest much time into making a crossover, calculating a box size etc. The problem with this is that you can't predict at all how the speaker will sound, because most of the times there's no information about them, aside from sensitivity, power and a (mostly useless) frequency response figure. My suggestion is: if you're on a budget and have some of these left over, use them. Do not try to make a ported box as most of the time it will sound bad! Make a reasonably sized closed box and they will play nicely. Just don't expect much bass out of them. For a rough estimate of how much volume you need, check this website (it's in german, but google translate can help :) )

To get the best results possible, you should buy hi-fi (non car audio) speaker drivers with documented specifications (known as Thiele-Small parameters), frequency response and impedance charts. My recommendation for those who don't know much about crossovers and box design is to look for already made projects (either free or with paid build plans), as the big part of modelling and designing the crossover and box size/type is already done and tested and proven to be good. Otherwise, an easy starting point are "Full-range" speakers, usually in the 3-4 inch size: they cover most of the frequency spectrum so there isn't the need of a crossover(*), and some of them can also achieve decent bass when put in an adequate enclosure. You can design an enclosure based on the driver Thiele-Small parameters, by putting those into a box modelling software like WinISD or VituixCAD2 (or SpeakerBoxLite which is online).

Some recommendations i can give you about YouTube channels teaching box modelling and crossover making are Toid's DIY Audio, Impulse Audio, DIY Audio Guy. For already made projects, there are many websites to check; one of them is Paul Carmody's website.

I suggest getting speaker grills in case you want to bring the speaker around with you, as this will protect the drivers from unwanted encounters with external objects, which can damage them.

In case you plan to reuse already made speakers, all of the above can be skipped and you just need to open the box to put the electronics inside. Usually this just requires to unscrew and remove the bigger speaker in the box.

(*): usually there's always some kind of crossover needed to compensate for several diffraction related problems, but for a starter project those can be temporarily be left aside, as they require some crossover making knowledge.

We need to get to know these things as they're crucial to determine a baseline/goal in your speaker design.

Sensitivity, in decibels, indicates how loud a speaker will play, measured 1 meter from a microphone, when given 1W of power (or 2.83V of voltage, which isn't always 1W but depends on the speaker impedance; the formula is power=voltage^2 / impedance). If you have a sensitivity of 90dB, this means that sending 1 watt of power to the speaker will produce 90dB of sound pressure level (SPL). Now, if you double the input power, you increase the SPL by 3dB (93dB at 2 watts), double again and you add another 3 (96dB at 4 watts), again (99dB at 8 watts), and so goes on.

The general rule is: doubling the power increases by 3dB the SPL (10x the power increases the spl by 10dB); doubling the cone area also increases the SPL by 3dB; doubling the number of speakers combines the two above, so increases the SPL by 6dB (and doubes power consumption).

These calculations are useful for determining a "target maximum SPL" and determining how much power you really need, because more power means either less battery life, or need for a bigger and more expensive battery. As an example: if i want to reach 106dB SPL from a pair of 90dB speakers, i'll just need 2x10 watts of power (90dB + 10 brings power from 1 to 10W; having two speakers, we go from 100 to 106dB with 2x10W of power consumption); if the speakers have 87dB of sensitivity, i'll need 2x20W instead, if they're 84dB i'll need 2x40W and so goes on.

How much maximum SPL do i need? This depends on what you'll do with the speaker. Outdoor partys? Shoot for 100-105dB in compact designs, and more in bigger speakers. Indoor listening? You probably won't need more than 90-95dB, so speakers with less sensitivity are good.

About impedance: this is a parameter to consider together with the amplifier, as it will determine how much power the amplifier can put out. I wouldn't go above 8 ohms as this will complicate the battery system (higher voltages are required). Most speakers are between 4 and 8 ohms anyways.

The amplifier is the heart and brain of a portable speaker, so it must be choosen according to all the rest of the components.

Since this is a battery powered system, it's natural to choose class-D amplifiers as their efficiency is higher compared to other amplifiers, therefore battery runtime will be longer and there will be less heat generated.

There are plenty of boards that already include bluetooth, so for a simple design you can use them, otherwise you'll need a separate bluetooth module with a voltage regulator (as they usually run on 5 volts).

Having determined how much power you will need you can pick an amplifier that will provide it.

For portable designs my go-to is the TPA3116 /TPA3118, which can be found for cheap on chinese amplifier boards that have or don't have bluetooth; it can provide over 50 watts of power to 4 ohm speakers, and over 30 watts to 8 ohm speakers. The maximum power of these amplifiers, though, is determined on the input voltage (see the datasheet , go to the graph called: "output power vs supply voltage"), which is your battery voltage OR the voltage coming out of a (for example) boost regulator. As an example, at 14V a tpa3116 can put out around 20W in a 4 ohm speaker(1% thd, so not much distortion, ignore the 10% thd curve), but only 10W in a 8 ohm speaker. if you need more power, you need to have a higher voltage going to your amplifier (within the margins of that chip obviously). So, choosen a chip, from these curves you can determine the voltage going to the amplifier, therefore determining how you'll make the battery system.

Also about impedance: all these chips have a specified minimum impedance in the datasheet. For example you can't connect 2 ohm speakers to a normal stereo tpa3116 (called BTL mode), but you can do this in a mono tpa3116 (called PBTL mode). 2.1 amplifiers, for example, have the sub channel put in PBTL mode, so they support down to 2 ohms of impedance, while the stereo channels are limited to 4 ohms. The tpa3110, as another example, works only up to 16V when using 4 ohm speakers (because of power limiting), this is another thing you can see from power vs voltage curves. So always check the datasheet first!

Other chips available in this category are the tpa3110(lower power but really similar to 3116, less background noise), tda7492, tda7492p, tda7498/7498p (for higher power)

Some good boards i tested: this simple tpa3118 stereo bluetooth, this tpa3116 with also a subwoofer channel, tone regulation and high pass for the left and right channel (in case you want to make a 2.1 system), these various tpa3116-based boards

P.S.: if building an ammo-box speaker, consider soldering an external antenna to the existing one, as the metal works as a Faraday Cage, thus reducing Bluetooth range. I used one from an old 2.4GHz Wi-Fi modem, it works okay since Bluetooth is also 2.4GHz (RF experts please don't get mad at me).

The last thing you need to design is the battery system.

My choice is to use Li-ion batteries: compared to lead acid they're lighter and don't need to stay always charged (lead acid batteries degrade when left discharged), and they require less cells to get a certain voltage compared to Ni-MH, while also being much more energy dense than Ni-CAD and having zero memory effect.

Since we are working with lithium cells, a quick disclaimer is necessary: never short-circuit li-ion cells, submerge them in water, heat them up, generally mistreat them: they really don't like being mistreated and can explode.

The most widespread form of Li-ion battery is the 18650 cell, used in vapes, laptop batteries and other electronics. These come with useful datasheets that indicate their current ratings (continuous and peak) which we'll use to choose the most adequate cells.

A bit of clarification about series/parallel connections. In a series connection you connect the positive of a battery to the negative of another; the voltage measured between the non connected positive and negative is double the voltage of a cell. In a parallel connection you connect the positives together and the negatives together, and you measure the same voltage as a single battery. The battery pack can be represented by the "xSyP" code: x is the number of series modules, y is the number of cells in parallel that make each module. A 4s1p battery has 4 modules in series, and each module is made of 1 cell (4 cells in total). a 5s2p battery has 5 modules in series, each module is made by 2 cells in parallel (10 cells in total).

Once you have determined how much power your amplifier will put out and what voltage you need to give to it, you have to determine how many cells you have to put in series to achieve that voltage: given your target voltage you just divide it by 3.7 and you'll get a rough number of series cells (or modules) needed. For example, if my target is around 14V, i'll need 4 cells in series. Note that the maximum voltage will be higher, as cells get to 4.2V when fully charged, so be sure the amplifier can tolerate that higher voltage.

To determine the current rating of the cells and be on the safe side, determine the total power of the amplifier when the cells are all charged (4.2V for each cell: for 4 cells in series it is 16.8V) and divide it by this voltage. For a tpa3116, at 16.8V you get 2x30W into 4 ohms, so 60W total. 60/16.8 gives 3.57A of current. Now the amplifier is not 100% efficient, so you can either get the efficiency from the datasheet or estimate a conservative 80-85% for a class-D. Divide the current by the efficiency and you get the real maximum current, in our example 4.5A. So theoretically any cell with a discharge rate above 4.5A will work. In practice, oversizing is important because the cell always has a voltage drop, which is high when approaching its maximum current capability, and can cause distortion. For a project with a stereo tpa3116 and a 4s battery, i'd go with cells with over 6-7A of discharge current. For higher power projects, get higher current cells, or make multi-cell modules that you'll then put in series.

You can now calculate how many watt-hours the battery has: simply multiply the nominal voltage of a single cell by its capacity (different than the current capability, it's indicated in mAh) by the total number of cells. If we have 4 cells in series, each cell with 3500mAh of capacity, we will have 3.7V * 3.5Ah * 4 = 51.8Wh.

How long will my battery last? Considering you are never using full power constantly (see music crest factor), you can assume the average power consumption to be 10% / 15% of the maximum power of your amplifier. So, to get the hours of runtime, you divide the watt-hours of your battery by 15% of the total output power, so in our case, considering a 51.8Wh battery and 50W total output power, we should get around 51.8 / (60*0.15) = 5 hours and 45 minutes (5.75) at full volume. You can use these calculations to determine roughly how many Ah you need for your battery.

When building a battery pack, be sure to always use batteries of the same exact model, and never mismatch!

Always get batteries from reliable sellers, as fake batteries don't match their claims and can be hazardous.

If you want to reuse old laptop/drill batteries, try to get informations about them (they usually have the model number printed on the cells), and test their capacity with something like a liitokala lii-500 or an opus bt-c3100 (single cell chargers and testers). Try to match them as good as possible, do not use batteries that have big differences in capacity, especially if you put them in parallel (avoid doing this with old mismatched batteries)

How do i keep the batteries together and connect them? I like to use battery cases with spring terminals, othwerise either you solder the batteries (not recommended) or spot weld them.

After choosing the number of batteries needed, you'll need a protection circuit, called BMS: this avoids overcharging, overdischarging, and short circuit; i personally always buy BMS circuits with balance charging as it keeps the cells always at the same voltage while charging (they are "balanced"), like this one . The "xS" means how many series "modules" you have: if you have four cells in series, your pack is a 4S1P pack, therefore you need a 4S BMS. If you have 5 cells in series then it's a 5S and you need a 5S BMS. Always check if the BMS has correct voltages, overvoltage protection (usually 4.3V per cell, not more). Get a BMS that can support the maximum current you have determined before.

Finally, you'll need a proper charger. Look for a charger (not a power adapter) that can do "cc-cv" and that is designed for li-ion batteries and is suitable for your pack (if you have a 5s pack, get a 5s charger and so on). Choose its maximum charging current depending on the maximum charging current of your cells: lower or equal is good, higher is never recommended. I personally use this for 4s1p batteries; there are also some chargers with a type c connector, like this one.

Now that everything is ready, you can wire the electrical system up. I have attached the scheme of the complete system with an on/off switch and a barrel jack for attaching the external charger (i usually use a 2.1x5.5 barrel jack; the internal tip is positive, the external ring is negative; be sure to check your polarity with a multimeter because some chargers may haver it inverted). I also have attached a scheme for a 4s battery attached to the BMS.

You can get creative and add what you want: LEDs? Vu meters? USB chargers? The sky is the limit! I personally always add a capacity indicator like this , or also this one.

While making the electrical system, put everything in your speaker box. In case your speakers share the same space as your electrical system, seal every possible air leak with silicon or hot glue, as this is important to avoid noise and performance losses, both in sealed or ported systems.

If you made it through all my instructable and made your own portable speaker, congratulations, YOU made it!

Now your favourite tunes will follow you wherever you go, and you can proudly answer "I made it" to those answering where you got that unique speaker.